Ozone control catalyst compositions

ABSTRACT

A highly efficient novel catalyst composition for ozone removal is made from 
     (a) metallic silver deposited on a relatively high surface area metal substrate and 
     (b) a composite of a relatively high surface area zirconia and oxides of manganese.

This application is a continuation-in-part of U.S. application Ser. No.944,836, filed Sept. 22, 1978, now U.S. Pat. No. 4,173,549.

BACKGROUND OF THE INVENTION

Ozone is generated for use in many chemical processes requiringreactions to be carried out in the presence of a strong oxidant. Ozoneis also generated as an undesired by-product in other processes, e.g.during use of certain electronic equipment such as electronicphotocopying machines. Because of the toxic and irritating effects onhumans, any ozone present in the process waste effluents must bedestroyed before said effluents can be released to the environment. Theremoval of ozone from the exhaust streams of the aforementionedprocesses to acceptable levels are not too difficult and can be achievede.g. by thermal destruction, by adsorption employing activated carbon,or by catalytic destruction using e.g. activated carbon-supported silveror copper catalyst.

The efficient control of ozone in the environment is, however, a problemin certain situations, where space and/or energy limitations precludethe use of the aforementioned conventional ozone destruction methods.Specifically, in order to conserve fuel on long distance flights, modernaircraft fly at such high altitudes, that they often penetrate the ozonelayer of the earth's atmosphere. Ozone is thereby introduced with thepressurized air into the cabin, causing discomfort and potential healthhazards to passengers and flight crew. The pressurized fresh cabin air,which is provided from an intermediate compression stage in the ramjetengines, is introduced at very high rates, typically in the order of 5pounds of air per second in larger commercial passenger planes. The useof activated carbon as an adsorbent or catalyst support for purificationof the air is not a desirable solution, since the weight (approximately400 lbs) and, more importantly, the space requirements of thepurification medium to treat such large quantities of air would beexcessive, and would necessitate major changes in the design of the aircirculation system. Another disadvantage of activated carbon is itsrelatively short service life at the delivery temperatures of thecompressed air (typically between 300 and 400° F.). Furthermore,activated carbon is quite brittle and is prone to disintegration duringperiods of high turbulence. Dust produced in such situations would beentrained in the purified air being supplied to the cabin and wouldrequire additional equipment for removal.

The use of higher temperatures, e.g. by taking the air from a rear stageof the engine, to destroy the ozone is technically feasible, but isenergy consuming, thus significantly negating the fuel savings achievedby flying at the higher altitudes.

It is therefore an object of the present invention to provide a novelefficient catalyst composition for the reduction of ozone levels.

It is another object of the invention to provide a catalyst and a devicefor the destruction of unwanted ozone with minimum energy requirementswhich is lightweight and operates with minimum pressure drop and atlower temperatures.

It is a further object to provide an ozone control device which can beinstalled in the pneumatic duct of an airplane.

Further objects of the invention will become apparent from a reading ofthe specification and appended claims.

THE INVENTION

In accordance with the invention there is provided a catalystcomposition which comprises

(a) metallic silver deposited on a relatively high surface area metalsubstrate, and

(b) a composite of a relatively high surface area zirconia and oxides ofmanganese.

The relatively high surface area metal substrate can be a particulatemetal, having an average particle size of from about 0.2 to about 10 mmwith roughened surface. The preferred metal substrates are those havinga continuous structure of relatively high surface area, such as fibrousfelts, fine wire mesh net cloths and foams. Generally, materials withrelative densities of from about 2 to about 40 percent (based on thedensity of the solid metal) are suitable in this invention. The mostpreferred materials are those having relative densities in the rangefrom about 2 to about 10 percent. These materials are all commerciallyavailable.

Illustrative examples of suitable metals are nickel, aluminum, zinccoated aluminum, copper, tungsten, chromium and molybdenum. The use ofcopper as the metal substrate has been disclosed in copending patentapplication Ser. No. 944,836, filed on Sept. 22, 1978, now U.S. Pat. No.4,173,549.

The silver is deposited on the metal substrate, illustratively a nickelsubstrate, to a thickness ranging from about 0.001 to about 10 mils,preferably from about 0.1 to about 1 mil. The deposition can be carriedout using any conventional plating process including electrolyticplating. Applicants have found, however, that a particularlyadvantageous method of forming an adherent silver coating onto the metalsubstrate is one where the metal is first pretreated with an ammoniacalsolution for removal of surface metallic oxides and for etching of thesurface. In a subsequent step the etched metal is treated with a solublesilver salt solution to cause replacement deposition of silver onto theroughened surface of the metal substrate.

In the aforementioned pretreatment step the concentration of theammoniacal solution is typically maintained between about 1 and about 15weight percent of NH₃. The metal, illustratively nickel, substrate isimmersed in the solution for a period of about 10 to about 100 secondsat room temperature, after which it is rinsed one or more times withwater. Preferably deionized water is used at least for the last rinse.

The pretreated substrate still having wetted surfaces is then immersedfor a period of 0.5 to 50 minutes into an aqueous silver salt solutioncontaining from about 0.01 to about 100 g/liter of soluble silver metal,preferably from about 0.1 to about 10 g/liter, and maintained at roomtemperature, although higher temperatures can also be used. Theresulting silver plated metal, illustratively nickel, is then rinsedwith water one or more times, preferably with deionized water afterwhich it is dried in an oven.

The zirconia can be a particulate zirconia having an average particlesize in the range from about 0.2 to about 10 mm. The preferred zirconiacompounds are those having a continuous structure of relative highsurface area, such as fibrous felts and woven or knit cloths. Generallymaterials having relative densities of from about 2 to about 40 percent(based on the density of solid zirconia) are suitable in this invention.The most preferred materials are those having relative densities in therange from about 2 to about 20 percent. The zirconia is usuallystabilized with small additions of other oxides such as silica, ferricoxide, titania, yttria, calcium oxide to improve theirthermal-mechanical properties. These materials are all commerciallyavailable.

The composite of zirconia and oxides of manganese is suitably preparedby first immersing or spraying the zirconia with a solution of a solublepermanganate, such as sodium or potassium permanganate, or of a solubledivalent manganese compound such as manganous nitrate, and then dryingthe treated zirconia. The application treatment can be carried out atroom temperature or higher temperatures using a solution containing fromabout 0.01 to about 1 gram mols/liter of the manganese compound.

The oxides of manganese are formed by heat treating any of theaforementioned zirconia/manganese compounds at a temperature from about120° F. to about 450° F. This heat treatment can be carried out afterthe catalyst components are assembled into the catalyst composition, andeven later, i.e. during use of the catalyst composition, if the ozoneremoval is carried out at temperatures of at least 200° F.

The concentration of the oxides of manganese should be from about 0.1 toabout 10 percent based on the weight of the zirconia, preferably between0.1 and about 0.5 percent.

Composites of zirconia/manganese oxides can also be prepared by anyother suitable method, e.g. by first forming a coprecipiitate of eitherhydroxides or carbonates by the action of respectively an alkali or acarbonate upon a solution of a zirconium salt and a manganese salt or apermanganate. The coprecipitate is then filtered, dried, calcined,preferably in the presence of stabilizers, to improve thethermal-mechanical properties, and shaped into particles, fibers, felts,cloths, etc.

The two components, i.e. (a) the silver coated metal and (b) thezirconia treated with the manganese compound (before or after heattreatment) are combined to provide a volumetric ratio of components (a)and (b) from about 1.5 to about 100:1, preferably from about 1:2 toabout 50:1. When they are in particulate or fibrous form, the componentsmay be mixed together, or arranged in alternate layers, separated, ifdesired, by retaining screens.

The catalyst mixture is suitably contained in a walled device, e.g. alength of stainless steel pipe, provided with retaining screens at theends to prevent escape of catalyst with the ozone containing streampassing through the device.

However, the preferred catalyst compositions are those where thecomponents are not subject to any significant entrainment by the gasbeing treated, even under conditions of severe vibration or turbulence.Thus, it is preferred to use the metal and the zirconia in extendedshapes or forms such as screens, felts, foams, cloths, woven nets asbase materials for the two components.

Catalytic devices can easily be constructed by arranging the silvercoated metal wire, felt or foam and the manganese treated zirconia feltor cloth in alternate layers within a suitable housing having gas inletand outlet means. The layers can be arranged angularly, e.g.perpendicularly, to the direction of the gas flow or in the samedirection as that of the gas flow. These arrangements can be made, e.g.by cutting discs or wafers of equal areas from the two materials andstacking them in alternate layers or by placing two rectangular piecesof the two materials one on the other and winding them around a mandrelin a cylindrical fashion.

The catalyst compositions of this invention are highly effective inremoving ozone from gas streams even at very high space velocities,(weight or volume of gas treated per weight or volume of catalyst). Infact, their efficiencies, when compared either on a weight or avolumetric basis, are superior to those of the prior art catalysts aswell as to the efficiencies of either of the components of thecompositions. In addition, the compositions of this invention are notsubject to significant deterioration by either heat or vibratorymovements and have a superior service life. Furthermore, they offer verylittle pressure resistance, i.e. the pressure drop is minimal in use.Because of the above-mentioned desired characteristics and the inherentlow requirements of energy, space and weight for purification of largequantities of ozone-containing gases, the catalyst compositions areparticularly useful for installation in existing pneumatic ducts onaircraft. However, the catalyst compositions are also useful for thecontrol of ozone levels in any gaseous stream, which needs to bepurified before being vented to the environment.

The following examples are provided to illustrate the invention.

EXAMPLE 1

A cylindrical laminar catalytic device having a diameter of 2 inches anda height of 1 inch was prepared by alternating layers of permanganatetreated zirconia felt with silver coated copper foam. The thickness ofeach of the 5 circular zirconia layers was 5/100 of an inch and that ofeach of the 4 interspaced circular copper layers was 3/16 of an inch.

Before the assembly the zirconia circular wafers had been cut from azirconia felt, having a relative density of 4 percent (8 percent yttriastabilized, type ZYF-50 obtained from Zircar Products, Inc., Florida,N.Y.). Then they were soaked in a bath containing a 0.1 M solution ofpotassium permanganate until thoroughly wetted and finally air dried.

The circular wafers of copper were cut from a copper foam having arelative density of about 15 percent (type 55P copper Foametal obtainedfrom Hogen Industries, Willoughby, Ohio).

The wafers were pretreated for 15 seconds in a bath maintained at roomtemperature and prepared from equal volumes of concentrated ammoniumhydroxide and water. The wafers were then rinsed thoroughly first withtap water and then with deionized water. After shaking water out of thefoam wafers, they were immersed in a 0.01 M silver nitrate bath forabout 4 minutes at room temperature. After removal from the bath, excesssolution was shaken out of the wafers, which were then rinsed withdeionized water and dried in an oven at 75° C. overnight.

The catalytic device was installed in a pipe and its efficiency wastested at 300° F. and 400° F. by passing air containing 1.5 ppm (byvolume) of ozone at velocities ranging between 200 and 800 ft/minthrough the device. At 300° F. the ozone removal was 86-98 percent, andat 400° F. the ozone removal was 92-99 percent. The efficiencies did notappear to be velocity dependent.

EXAMPLE 2

A cylindrical catalytic device (10 inch diameter, height 2.5 inches) wasprepared by placing a length of permanganate treated zirconia tricotweave cloth onto a length of silver coated copper foam and winding thetwo layers tightly in a spiral fashion around a 2.5 inch long mandrelhaving a diameter of one inch. The mandrel was fabricated from copperpipe closed off at each end with 316 stainless steel cap affixed withsilver solder.

The zirconia cloth was a tricot knit zirconia having a relative densityof 15 percent and a thickness of 0.015 inches (8 percent yttriastabilized, type ZYK 15 obtained from Zircar Products, Inc.). Thepermanganate treatment of the cloth prior to assembly was carried out inaccordance with the method set forth in Example 1.

The copper foam substrate used in this example had a relative density ofabout 2-4 percent and a thickness of 1/8 inch (type 30P Copper Foametalobtained from Hogen Industries). The silver coating procedure wasessentially the same as that of Example 1 except that the immersion timein the silver nitrate bath was 1.5 minutes.

The efficiency of the catalytic device was tested by passing ozonecontaining air at high rates through the cylinder. The results of thetests are shown below in Table I.

                  TABLE I                                                         ______________________________________                                                                            Press.                                    Air Rates      Ozone - ppm                                                                              Efficiency                                                                              Drop                                      Temp °F.                                                                      lbs/sec ft/min  Inlet                                                                              Outlet                                                                              %       psi                                 ______________________________________                                        345    2.17    2337    1.49 0.05  97      2.00                                405    2.14    2362    1.50 0.03  98      2.05                                ______________________________________                                    

EXAMPLE 3

A cylindrical laminar catalytic device having a diameter of 2 inches anda height of 1 inch was prepared by alternating layers of permanganatetreated zirconia felt with silver coated nickel foam. The thickness ofeach of the 5 circular zirconia layers was 5/100 of an inch and that ofeach of the 4 interspaced circular nickel layers was 3/16 of an inch.

Before the assembly the zirconia circular wafers had been cut from azirconia felt, having a relative density of 4 percent (8 percent yttriastabilized, type ZYF-50 obtained from Zircar Products, Inc., Florida,N.Y.). Then they were soaked in a bath containing a 0.1 M solution ofpotassium permanganate until thoroughly wetted and finally air dried.

The circular wafers of nickel were cut from a nickel foam having arelative density of about 4 percent and a porosity of 30 pores per inch.

The wafers were pretreated for 15 seconds in a bath maintained at roomtemperature and prepared from equal volumes of concentrated ammoniumhydroxide and water. The wafers were then rinsed thoroughly first withtap water and then with deionized water. After shaking water out of thefoam wafers, they were immersed in a 0.01 M silver nitrate bath forabout 4 minutes at room temperature. After removal from the bath, excesssolution was shaken out of the wafers, which were then rinsed withdeionized water and dried in an oven at 75° C. overnight.

EXAMPLE 4

A cylindrical catalytic device (10 inch diameter, height 2.5 inches) wasprepared by placing a length of permanganate treated zirconia tricotweave cloth onto a length of silver coated zinc coated aluminum foam andwinding the two layers tightly in a spiral fashion around a 2.5 inchlong mandrel having a diameter of one inch. The mandrel was fabricatedfrom zinc coated aluminum pipe closed off at each end with 316 stainlesssteel cap affixed with silver solder.

The zirconia cloth was a tricot knit zirconia having a relative densityof 15 percent and a thickness of 0.015 inches (8 percent yttriastabilized, type ZYK 15 obtained from Zircar Products, Inc.). Thepermanganate treatment of the cloth prior to assembly was carried out inaccordance with the method set forth in Example 1.

The zinc coated aluminum foam substrate used in this example had arelative density of about 5 percent and a porosity of about 33 pores perinch. The silver coating procedure was the same as that of Example 2.

EXAMPLE 5

A catalytic device was prepared by sandwiching a piece of silver-coatednickel foam, 1 inch long by 1/2 inch wide by 1/8 inch thick, between two1 inch by 1/2 inch pieces of permanganate treated zirconia cloth. Thesilver-coated nickel foam substrate assembly was prepared by copperplating the nickel foam using an electroless copper solution and thesilver plating the copper plated nickel foam by the procedure describedin Example 2.

EXAMPLE 6

A catalytic device was prepared exactly as described in Example 5 exceptthat a zinc coated aluminum foam was used in place of the nickel foamdescribed in Example 5.

The efficiency of the catalytic devices of Examples 5 and 6 were testedby passing ozone containing air at high rates through the catalyticdevices. A silver coated copper foam containing device was similarlyprepared and tested. The results of the tests are shown in Table II.

                  TABLE II                                                        ______________________________________                                                                        Effic-                                        Temp        Air Rate Ozone-ppm  iency Pressure                                Metal   °F.                                                                            ft/min   Inlet                                                                              Outlet                                                                              %     Drop-psi                            ______________________________________                                        Copper  350     4400     1.82 0.15  91    .29                                 Aluminum                                                                              350     4400     1.75 0.85  51    .19                                 Nickel  350     4400     1.71 0.20  88    .25                                 ______________________________________                                    

What is claimed is:
 1. A catalyst composition for the decomposition ofozone which comprises:(a) metallic silver deposited on a relatively highsurface area metal substrate, and (b) a composite of a relatively highsurface area zirconia and oxides of manganese.
 2. The catalystcomposition of claim 1 wherein the metal substrate is selected fromparticulate metal, foamed metal, metal felt and metal wire mesh.
 3. Thecatalyst composition of claim 1, wherein the zirconia is selected fromparticulate zirconia, zirconia fibers, fibrous zirconia felt or zirconiacloth.
 4. The catalyst composition of claim 1, wherein the thickness ofthe silver deposit is between about 0.001 and about 10 mils.
 5. Thecatalyst composition of claim 4, wherein said thickness is between about0.1 and about 1 mil.
 6. The catalyst composition of claim 1, wherein theconcentration of the oxides of manganese is between about 0.1 and about10 percent based on the weight of the zirconia.
 7. The catalystcomposition of claim 6, wherein the concentration of oxides of manganeseis between about 0.1 and about 0.5 percent based on the weight of thezirconia.
 8. The catalyst composition of claim 1 wherein the volumetricratio of components (a) and (b) is maintained between about 1:5 andabout 100:1.
 9. The catalyst composition of claim 8, wherein saidvolumetric ratio is between about 1:2 and about 50:1.
 10. The catalystcomposition of claim 1 wherein the relative density of the metalsubstrate ranges from about 2 to about 40 percent.
 11. The catalystcomposition of claim 1 wherein the relative density of the zirconiaranges from about 2 to about 40 percent.
 12. The catalyst composition ofclaim 1 wherein the relative density of the metal substrate ranges fromabout 2 to about 10 percent.
 13. The catalyst composition of claim 1,wherein the relative density of the zirconia ranges from about 2 toabout 20 percent.
 14. The catalyst composition of claim 1, wherein thezirconia is stabilized with an inorganic oxide.
 15. The catalystcomposition of claim 14, wherein the inorganic oxide is yttria.
 16. Thecatalyst composition of claim 1, wherein the metal substrate is a nickelsubstrate.
 17. The catalyst composition of claim 1, wherein the metalsubstrate is an aluminum substrate.
 18. The catalyst composition ofclaim 1, wherein component(a) is prepared by a process comprising: (i)etching the metal substrate in a solution of ammonium hydroxide; (ii)rinsing the etched metal substrate with water, and subsequently (iii)treating it with a solution of a soluble silver salt to deposit anadherent coating of metallic silver on said metal substrate.
 19. Thecatalyst composition of claim 1, wherein component (b) is prepared byimpregnating the zirconia with a permanganate solution, removing excesssolution from the impregnated zirconia and heating said impregnatedzirconia to convert the permanganate to oxides of manganese.
 20. Thecatalyst composition of claim 19, wherein said heating step is conductedduring the use of the catalyst for ozone removal.
 21. A catalytic devicefor ozone removal which comprises alternate layers of(a) metallic silverdeposited on a relatively high surface area metal substrate and (b) acomposite of a relatively high surface area zirconia and oxides ofmanganese.
 22. The catalytic device according to claim 21 wherein thealternate layers are arranged in laminar fashion.
 23. The catalyticdevice according to claim 21, wherein the alternate layers are arrangedin spiral fashion.
 24. The catalytic device according to claim 21,wherein the metal substrate is a metal foam and the zirconia is afibrous zirconia felt.
 25. The catalytic device according to claim 21,wherein the substrate is a metal foam and the zirconia is a woven cloth.26. The catalytic device according to claim 21, wherein the metalsubstrate is a nickel foam.
 27. The catalytic device according to claim21, wherein the metal substrate is an aluminum foam.
 28. The catalyticdevice according to claim 21, wherein the thickness of the silverdeposit is between about 0.001 and about 10 mils, the concentration ofthe oxides of manganese is between about 0.1 and about 10 percent basedon the weight of the zirconia, and the volumetric ratio of components(a) and (b) is maintained between about 1:2 and about 50:1.